1. Field of the Invention
This invention relates to optical devices such as photo-interrupters that are utilized for bar code reading and object detection.
2. Description of Related Art
FIGS. 23 and 24 show a conventional reflective-type photo-interrupter 80 equipped with a light-emitting unit and a light-sensing unit. The light-emitted from the light-emitting unit is reflected off an object surface and is then detected by the light-sensing unit. In this conventional photo-interrupter 80, light-emitting chip 2 and light-sensing chip 3 are mounted to a head 4 by some method such as gold wire bonding. This photo-interrupter is further provided with a lens 5 which focuses a read-beam emitted from light-emitting element 2 on the object surface and converges a reflected-beam from the object surface on the light-sensing element 3, and a case which houses various members of the unit and blocks ambient light from coming into the unit, and a lens cover 18 for protecting the lens 5.
When reading a bar code 6 using reflection-type photo-interrupter 80 of the aforementioned construction, the bar code 6 is moved above the reflective-type photo-interrupter 80, or the reflective-type photo-interrupter 80 is moved above bar code 6, thereby reading the bar code 6.
When the read-beam is converged to a certain spot on a white area of the bar code 6, the amount of the reflected-beam is large and therefore the light-sensing chip 3 receives a greater amount of light. This results in an increase in the photo-current generated by the light-sensing chip 3. Also, when the read-beam is focused at a certain spot on a black area of the bar code 6, the amount of the reflected-beam is small, resulting in a relative decrease in the photo-current generated by light-sensing chip 3. In this manner, the bar code pattern is converted into a varying photo-current generated by the light-sensing chip 3.
In order to realize a small sized photo-interrupter, the distance from the light-emitting chip 2 and light sensing chip 3 to the bar code 6 is preferably as short as possible.
The focal length of the lens 5 can be shortened by decreasing the radius of curvature of the lens 5, thereby decreasing the distance from the light-emitting chip 2 and light-sensing chip 3 to the bar code 6. However, the radii of curvature of the lens 5 smaller than a certain value cause larger aberrations of the lens 5, making it difficult to distinguish between the white and black areas as well as reducing the depth of focus. A reduced depth of focus makes it difficult to accurately read the bar codes when the bar codes are moving in the direction of the optical axis of the read-beam. Therefore, the distance from the light-emitting chip 2 and light sensing chip 3 to the bar code 6 cannot be shorter than a certain minimum value. This limitation prevents package reduction of an apparatus incorporating photo-interrupter 80.
Also, for a reflection-type photo-interrupter such as the one shown in FIGS. 23 and 24, the light-emitter and light-sensor are placed in a mirror relation for ease of manufacture. With this construction, the light directly reflected (referred to as direct reflection hereinafter) from the bar codes enters directly the light-sensing chip. The intensity of the direct-reflection is more sensitive to the luster of the medium on which the bar code is printed than to the black and white areas of the bar code. Thus, the bar codes on a very lustrous object cannot be read properly. In order to accurately read such bar codes, the sensor is tilted relative to the bar code as shown in FIG. 25 so that the reflected-beam contains a lesser amount of direct-reflection with an increased ratio of scattered light to the direct-reflection.
However, a tilted photo-interrupter occupies a larger space within the housing of the interrupter, presenting a problem in assembling it into the apparatus. Thus, incorporating such a photo-interrupter in the apparatus is an obstacle to miniaturizing the apparatus.